1. Field of the Invention
The present invention relates to a thermal flow sensor for measuring a flow rate by means of hot wires, and more particularly to a thermal flow sensor capable of reducing a turbulent flow which occurs in a passage in order to stabilize measurement output.
2. Description of Related Art
As one of conventional thermal flow sensors for measuring a flow rate by using hot wires, there is a thermal flow sensor provided with a measuring chip manufactured by a semiconductor micro-machining technology as a sensor part. An example of this type of thermal flow sensor is shown in FIG. 23. In a thermal flow sensor 101 in FIG. 23, a gas to be measured is caused to flow in an inlet port 102, is made into a laminar flow by a laminar flow mechanism 103, and is caused to pass through a measuring passage 104 and flow out through an outlet port 105. For measurements of the flow rate of the gas, a measuring chip 111 connected to an electric circuit 106 is arranged in an exposed state in the measuring passage 104.
In this regard, the measuring chip 111 is constructed, as shown in FIG. 24, of a silicon chip 116 on which an upstream thermo-sensor 112, a heater 113, a downstream thermo-sensor 114, and an ambient temperature sensor 115 (these sensors 112 through 115 correspond to “hot wires”) and others are formed by a semiconductor micro-machining technology.
In the thermal flow sensor 101, six electrodes D1, D2, D3, D4, D5, and D6 are provided on the silicon chip 116 of the measuring chip 111 in FIG. 24 so that the upstream thermo-sensor 112, the heater 113, the downstream thermo-sensor 114, and the ambient temp. sensor 115 are individually connected to the electric circuit 106 by wire bonding (W in FIG. 23) using those six electrodes D1-D6.
In such thermal flow sensor 101, while the gas does not flow through the measuring passage 104, the distribution of temperatures of the measuring chip 111 in FIG. 24 is symmetrical with respect to the heater 113. While the gas flows through the measuring passage 104, on the other hand, the temperature of the upstream thermo-sensor 112 is decreased and the temperature of the downstream thermo-sensor 114 is increased. Accordingly, the distribution of temperatures of the measuring chip 111 in FIG. 24 becomes unsymmetrical according to the flow rate of the gas to be measured. At this time, the degree of such unsymmetry is outputted as a difference in resistance values between the upstream thermo-sensor 112 and the downstream thermo-sensor 114. Thus, the flow rate of the gas can be measured by the electric circuit 106.
For handling of semiconductor chips during a mounting work, vacuum suction is utilized. Therefore, a pressure sensor is conventionally used to check a suction state. In recent years, however, semiconductor chips have been reduced in size rapidly. For a chip of 0.5 mm square, for example, a suction orifice (nozzle) having a diameter of 0.5 or 0.3 mm is used. As a result, the pressure in the orifice varies between a sucking state and a non-sucking state, so that the pressure sensor could not check the suction state. Under the circumstances, there has been proposed a method of checking a suction state by detecting the flow rate of air flowing through the orifice.
However, the above mentioned thermal flow sensor 101 shown in FIG. 23 has a problem of slow response (about 1-2 sec.). As this reason, it is conceivable that the laminar flow mechanism 103 could not eliminate the turbulence occurring in the measuring passage 104 and therefore an integration process is performed on output signals to prevent the output signals from being influenced by the turbulence. There is also a problem that the thermal flow sensor 101 shown in FIG. 23 is too large to be used for checking the suction state. It is therefore difficult to utilize the thermal flow sensor 101 for a suction check.
To solve the problems of response and size, the applicant of the present invention has proposed a compact thermal flow sensor of quick response (50 ms) in Japanese patent application No. 2000-368801. This thermal flow sensor, which is compact in size and excellent in response, could be used appropriately for a suction check. In the thermal flow sensor proposed by the applicant in Japanese patent application No. 2000-368801, however, there is still a problem that the influence of a turbulent flow in a passage become larger as the measured flow rate becomes higher. In other words, there further occurs a problem that, as the measured flow rate becomes higher, measurement output becomes unstable due to the turbulent flow in the passage. Consequently, even the thermal flow sensor proposed in Japanese patent application No. 2000-368801 could not sufficiently perform an accurate suction check.